The present invention relates to a method of forming a coated layer of a fluorescent substance on an inner surface of a bulb of a fluorescent lamp.
Heretofore, when a fluorescent substance layer is formed on an inner surface of a bulb of a fluorescent lamp or a mercury lamp, an aqueous lacquer is prepared by dissolving a water soluble organic polymer and a surfactant in water and suspending a fluorescent substance and a small amount of binder for improving bond strength of the fluorescent substance particles on the surface of glass bulb in the lacquer to form a suspension of a fluorescent substance, and coating the suspension on the inner surface of the bulb and drying it in a desired condition and then, burning out the organic polymer and the surfactant by heating the bulb to form the fluorescent substance layer.
In general, colloidal alumina of fine powdery aluminum oxide is used as the binder for the fluorescent substance. Thus the colloidal alumina has significant effect for increasing bond strength of the fluorescent substance; however, the colloidal alumina has a characteristic that it is converted to have high activity for a gas absorption after heating in the bulb heat-treating step.
Accordingly, in the fluorescent substance layer formed by the conventional method, the binder in the fluorescent substance layer absorbs or adsorbs large amounts of moisture and carbon dioxide gas in air on the surface of the binder. The absorbed gases are not easily evacuated in the following evacuating step of evacuating the bulb whereby the absorbed gases remain as impurity gases in the bulb and the lamp characteristics such as luminous flux maintenance factor of the lamp are impaired.